CART Thermochemistry and Energy 1 — Questions and Answers
Question 1: What is the term for the heat released or absorbed during a chemical reaction at constant pressure?
- Enthalpy change (ΔH) (Correct answer)
- Entropy change (ΔS)
- Gibbs free energy (ΔG)
- Internal energy (ΔU)
Correct answer: Enthalpy change (ΔH)
Enthalpy change (ΔH) represents the heat flow in a reaction carried out at constant pressure.
Question 2: A reaction with a negative ΔH value is classified as:
- Endothermic
- Exothermic (Correct answer)
- Isothermal
- Adiabatic
Correct answer: Exothermic
A negative ΔH means the system releases heat to the surroundings, making the reaction exothermic.
Question 3: Which law states that the enthalpy of a reaction is the same whether it occurs in one step or multiple steps?
- Law of Conservation of Energy
- Hess's Law (Correct answer)
- Le Chatelier's Principle
- Avogadro's Law
Correct answer: Hess's Law
Hess's Law states that total enthalpy change is path-independent and can be calculated by adding individual step enthalpies.
Question 4: The specific heat capacity of water is 4.18 J/(g·°C). How much heat is needed to raise 50 g of water by 10°C?
- 418 J
- 209 J
- 2090 J (Correct answer)
- 4180 J
Correct answer: 2090 J
Using q = mcΔT: q = 50 g × 4.18 J/(g·°C) × 10°C = 2090 J.
Question 5: Which of the following processes is endothermic?
- Condensation of steam
- Freezing of water
- Combustion of methane
- Melting of ice (Correct answer)
Correct answer: Melting of ice
Melting requires heat input to break intermolecular forces, making it an endothermic process.
Question 6: Standard enthalpy of formation (ΔH°f) for an element in its standard state is:
- 1 kJ/mol
- –1 kJ/mol
- 0 kJ/mol (Correct answer)
- Varies by element
Correct answer: 0 kJ/mol
By definition, the standard enthalpy of formation of any element in its most stable standard state is exactly zero.
CART Thermochemistry and Energy 1
What is the term for the heat released or absorbed during a chemical reaction at constant pressure?